High solids precipitated calcium carbonate with copolymeric additive

ABSTRACT

The present invention relates to a process for producing an aqueous suspension of precipitated calcium carbonate, an aqueous suspension of precipitated calcium carbonate and a precipitated calcium carbonate obtained by the process, a product comprising the aqueous suspension of precipitated calcium carbonate or the precipitated calcium carbonate as well as its use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a U.S. national phase of PCT Application No. PCT/EP2016/074567, filed Oct. 13, 2016, which claims priority to European Application No. 15190284.8, filed Oct. 16, 2015.

The present invention relates to a process for producing an aqueous suspension of precipitated calcium carbonate, an aqueous suspension of precipitated calcium carbonate and a precipitated calcium carbonate obtained by the process, a product comprising the aqueous suspension of precipitated calcium carbonate or the precipitated calcium carbonate as well as its use.

Calcium carbonate is one of the most commonly used additives in the paper, paint and plastics industries. While naturally occurring ground calcium carbonate (GCC) is usually used as a filler in many applications, synthetically manufactured precipitated calcium carbonate (PCC) may be tailor-made with respect to its morphology and particle size allowing this materials to fulfil additional functions.

Commonly known PCC production processes including the steps of slaking quicklime with water, and subsequently precipitating calcium carbonate by passing carbon dioxide through the resulting calcium hydroxide suspension, produce only PCC slurries with low solids content. Therefore, these processes typically comprise a subsequent solids concentration step in order to obtain a more concentrated PCC slurry, for example, for shipping the PCC slurry. However, such additional solids concentration steps are energy-consuming and cost-intensive and require equipment such as a centrifuge, which is expensive and needs high maintenance. Furthermore, mechanical dewatering processes using centrifuges can destroy the structure of the formed PCC, for example, in case of clustered scalenohedral PCC.

WO 2011/121065 A1 discloses a process for preparing PCC comprising inter alia the step of preparing an aqueous suspension of PCC seeds by carbonating a suspension of calcium hydroxide in the presence of strontium hydroxide. A process for producing PCC, wherein the addition rate of the calcium carbonate slurry to the reaction vessel is such that a certain electrical conductivity is maintained in the reaction vessel, is described in EP 2 537 900 A1.

US 2011/158890 A1 describes a method to manufacture PCC involving the use of a comb polymer, which reduces the carbonation time of the PCC. A grinding agent for grinding coarse lime is disclosed in EP 0 313 483 A1. EP 2 447 213 A1 relates to the production of high purity PCC involving the step of slaking lime with an aqueous ammonium chloride solution.

WO 2013/142473 A1 relates to a process comprising the steps of slaking quick lime to obtain slaked lime, and subjecting the slaked lime, without agitation, without prior cooling in a heat exchanger, and in the absence of any additives, to carbonation with carbon dioxide gas to produce PCC. PCC production processes including additives are disclosed in U.S. Pat. Nos. 6,294,143, 5,232,678, and 5,558,850. A method for producing slaked lime by slaking lime with a polymer having anionic groups is described in JP 2008/074629 A. EP 0 844 213 A1 discloses a method of producing a precipitate of an alkaline earth metal compound involving the use of a dispersing agent.

WO 2010/018432 A1 discloses a process to prepare precipitated calcium carbonate implementing low charge acrylate and/or maleinate-containing polymers. A process for producing platy precipitated calcium carbonate involving the step of adding a polyacrylate to a suspension of calcium hydroxide prior to the completion of carbonation is described in WO 2005/000742 A1. WO 2004/106236 A1 relates to a process for producing platy precipitated calcium carbonate involving the step of adding a dry condensed phosphate additive to a suspension of calcium hydroxide prior to the completion of carbonation.

It is further known from applicants' unpublished application EP 14 166 751.9 that an aqueous suspension of precipitated calcium carbonate can be prepared by carbonating a milk of lime, which has been obtained by mixing water, a calcium oxide containing material, at least one water-soluble polymer having a molecular weight M_(w) in the range from 200 to 6 500 g/mol, and at least one slaking additive, wherein the calcium oxide containing material and the water are mixed in a weight ratio from 1:2.5 to 1:6. The at least one water-soluble polymer has the chemical structure of formula (I)

wherein n, m, and p are integers and at least one of n, m, or p is greater than zero and n+m+p is less than or equal to 70,

R₁ is H or CH₃,

R₂ is H or CH₃;

R₃ is —C(═O)—O—R₄ or —C(═O)—NH—R₄, wherein R₄ is a C₁ to C₂₀ alkyl group, a C₃ to C₂₀ cycloalkyl group and/or a C₆ to C₃₀ aryl group, being optionally substituted with one or more sulphonate groups, and wherein the cycloalkyl group and/or the aryl group comprises one ring or several rings, which are linked to each other, and

X is H and/or M, wherein M is Na, K, Li, Mg, and/or Ca, and wherein the structural units

are arranged randomly, regularly and/or in blocks.

Furthermore, reference is made to the applicant's unpublished application EP 15 157 025.6, which describes a process for producing an aqueous suspension of precipitated calcium carbonate, wherein a milk of lime is carbonated, which is obtained from mixing water, a calcium oxide containing material, and at least one cationic polymer.

Furthermore, reference is made to the applicant's unpublished application EP 15 177 344.7, which describes a process for producing an aqueous suspension of precipitated calcium carbonate, wherein a milk of lime is carbonated, which is obtained from mixing water, a calcium oxide containing material, and at least one depolymerized carboxylated cellulose having a molecular weight M_(w) in the range from 10 000 to 40 000 g/mol.

A method for preparing calcium carbonate using additives and precipitation agents is also known from KR100958593 B1. The method comprises an elution step of adding, into a container equipped with an agitator, a lime-based byproduct comprising steel-making slag or quick lime (CaO) dust, water at a ratio of 20 to 50 l per 60 to 100 g of the lime-based byproduct, at least one additive selected from the group consisting of sodium trimetaphosphate, sodium hexametaphosphate, sodium polycarbonate, ammonium polycarbonate, sodium polycarboxylate, formic acid, succinic acid, sucrose fatty acid ester, sodium citrate, ammonium citrate, and ammonium chloride in an amount of 0.01 to 10.0 parts by weight with respect to 100 parts by weight of the lime-based byproduct, and at least one precipitating agent selected from the group consisting of a cationic precipitating agent, an anionic precipitating agent, and a nonionic precipitating agent in an amount of 0.01 to 3.0 parts by weight with respect to 100 parts by weight of the lime-based byproduct and performing a mixing, to elute calcium ions; a precipitation step of standing the resulting mixture for a predetermined period of time after the completion of mixing the lime-based byproduct, water, the additive, and the precipitating agent in the elution step to precipitate the lime-based byproduct; a carbonation step of separating a clear supernatant eluate after the completion of the precipitation step and then feeding carbon dioxide into the eluate to cause a reaction until the eluate is at pH 9; and a calcium carbonate collection step of collecting calcium carbonate precipitated on the bottom after the completion of the carbonation step.

However, said methods have the drawback that an additive in combination with a precipitating agent has to be added to the calcium oxide comprising material. Furthermore, especially in KR100958593 B1, it is described that the obtained mixture is separated in a precipitated bottom portion and a clear supernatant eluate. The carbonating and the resulting precipitation of calcium carbonate is then only carried out on the obtained clear supernatant eluate in order to obtain a calcium carbonate product comprising less impurities. Thus, said method requires additional separation steps allowing a separation of solid and liquid phase during processing which results in a more time and cost-consuming production of precipitated calcium carbonate. Furthermore, it is to be noted that the precipitation agent is used in said method for adsorbing the slurry suspended in water to coagulate and precipitate the slurry through cross-linking, which enables a rapid solid-liquid separation. However, due to the following separation of the liquid and solid phases and carbonating of only the liquid phase, i.e. the clear supernatant eluate, the precipitation agent is not present in the carbonating step and is thus not used for the following precipitation of calcium carbonate.

WO 2007/067146 A1 is concerned with a method of producing precipitated calcium carbonate, wherein the carbonation of calcium hydroxide is performed in the presence of starch or carboxymethylcellulose.

In view of the foregoing, there is a continuous need for processes providing precipitated calcium carbonate, and especially those which allow the direct production of PCC suspensions with a high solids content without an additional separation or concentration step.

Accordingly, it is an object of the present invention to provide a process for producing a PCC suspension with a high solids content at an acceptable viscosity. It is also desirable that said process does not require any mechanical or thermal concentration step during processing. It is also desirable that said process does not require any separation step during processing, especially before the milk of lime is carbonated. It is also desirable that said process does not affect the kinetics of the carbonation step in a negative way and/or does not impair the crystallographic structure of the PCC.

The foregoing and other objects are solved by the subject-matter as defined herein in the independent claims.

According to one aspect of the present invention, a process for producing an aqueous suspension of precipitated calcium carbonate is provided, the process comprising the steps of:

-   -   i) providing a calcium oxide containing material,     -   ii) providing at least one copolymer of the following formula         (I)

-   -   wherein x, y and z are present in blocks, alternating or         randomly; x is >0 and at least one of y or z is >0 and the sum         of x+y+z is ≤150; R₁ represents hydrogen or a sulphonic         functional group; R₂ represents a heteroatom, optionally         substituted with an alkyl group, an alkenyl group, an heteroaryl         group and/or a polyalkoxylated group; R₃ and R₄ are         independently from each other a hydroxyl group, (O⁻M⁺) with M⁺         being a monovalent, divalent or trivalent cation, an O-alkyl         group comprising from 1 to 20 carbon atoms, an N-alkyl group         comprising from 1 to 20 carbon atoms and/or a polyalkoxylated         group,     -   iii) preparing a milk of lime by mixing water, the calcium oxide         containing material of step i), and the at least one copolymer         of step ii) to obtain a milk of lime, wherein the calcium oxide         containing material and the water are mixed in a weight ratio         from 1:1 to 1:12, and     -   iv) carbonating the milk of lime obtained in step iii) to form         an aqueous suspension of precipitated calcium carbonate.

According to another aspect, the present invention provides an aqueous suspension of precipitated calcium carbonate obtainable by the process according to the present invention.

According to a further aspect, the present invention provides a product comprising an aqueous suspension of precipitated calcium carbonate according to the present invention, preferably the product is an aqueous formulation, and more preferably is a coating formulation, a paper coating formulation, a pharmaceutical formulation, an agricultural formulation, a cosmetic formulation or an oral care formulation.

According to still another aspect, the present invention provides a precipitated calcium carbonate obtainable by the process according to the present invention.

According to a still further aspect, a product comprising precipitated calcium carbonate according to the present invention is provided, preferably the product is a paper, a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product or a pharmaceutical product, and more preferably the precipitated calcium carbonate is a dried precipitated calcium carbonate and the product is a plastic or a polymer composition.

According to still another aspect, a use of the aqueous suspension of precipitated calcium carbonate according to the present invention and/or precipitated calcium carbonate according to the present invention in paper, plastics, polymer compositions, paint, coatings, concrete, cosmetics, pharmaceutics and/or agriculture applications is provided, wherein preferably a dried precipitated calcium carbonate, more preferably a dried powder of precipitated calcium carbonate, is used in plastics and/or polymer compositions.

Advantageous embodiments of the present invention are defined in the corresponding sub-claims.

According to one embodiment, step iii) comprises the steps of: a1) mixing the at least one copolymer of step ii) with water, and a2) adding the calcium oxide containing material of step i) to the mixture of step a1); or b1) mixing the calcium oxide containing material of step i), and the at least one copolymer of step ii), and b2) adding water to the mixture of step b1); or c) mixing the calcium oxide containing material of step i), the at least one copolymer of step ii) and water simultaneously.

According to another embodiment, the process further comprises step v) of adding at least one slaking additive to process step iii), preferably the at least one slaking additive is selected from the group consisting of organic acids, organic acid salts, sugar alcohols, monosaccharides, disaccharides, polysaccharides, gluconates, phosphonates, lignosulphonates, and mixtures thereof.

According to yet another embodiment, the milk of lime obtained in step iii) has a Brookfield viscosity from 1 to 1 000 mPa·s at 25° C., more preferably from 5 to 800 mPa·s at 25° C., and most preferably from 10 to 500 mPa·s at 25° C.; and/or the suspension of PCC obtained in step iv) has a Brookfield viscosity of less than or equal to 3 000 mPa·s at 25° C., more preferably less than or equal to 2 500 mPa·s at 25° C., and most preferably less than or equal to 600 mPa·s at 25° C.

According to one embodiment, the suspension of PCC obtained in step iv) has solids content of at least 15 wt.-%, preferably from 15 to 70 wt.-%, more preferably from 19 to 60 wt.-%, even more preferably from 21 to 50 wt.-% and most preferably from 24 to 42 wt.-%, based on the total weight of the suspension.

According to another embodiment, the temperature of the water, which is used in mixing step iii), is adjusted to be in the range from more than 0° C. and less than 100° C., preferably from 1° C. to 85° C., more preferably from 2° C. to 70° C., even more preferably from 30° C. to 65° C., and most preferably from 35 to 55° C.; and/or the temperature of the milk of lime obtained in step iii), which is employed in step iv), is adjusted to be in the range from 20° C. to 65° C., and preferably from 30° C. to 55° C.

According to yet another embodiment, the at least one copolymer of step ii) is a compound of the following formula (II)

wherein x and y are present in blocks, alternating or randomly; x and y are >0 and the sum of x+y is ≤150; R₁ represents hydrogen or a sulphonic functional group; R₂ represents a heteroatom, optionally substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group.

According to one embodiment, the at least one copolymer of step ii) is a compound of the following formula (III)

wherein x and z are present in blocks, alternating or randomly; x and z are >0 and the sum of x+z is ≤150; R₁ represents hydrogen or a sulphonic functional group; R₃ is a hydroxyl group, (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation, an O-alkyl group comprising from 1 to 20 carbon atoms, an N-alkyl group comprising from 1 to 20 carbon atoms and/or a polyalkoxylated group; and M⁺ being a monovalent, divalent or trivalent cation.

According to another embodiment, the molar ratio of x to y and/or z in the at least one copolymer of step ii) [x:y and/or z] is from 10:1 to 1:2, preferably from 5:1 to 1:2 and most preferably about 1:1, 2:1 or 3:1.

According to yet another embodiment, the at least one copolymer of step ii) has a molecular weight M_(w) in the range from 500 to 100 000 g/mol, preferably from 1 000 to 50 000 g/mol, more preferably from 2 000 to 20 000 g/mol and most preferably from 2 500 to 10 000 g/mol; and/or, provided in a water solution, has a Brookfield viscosity of less than or equal to 5 000 mPa·s at 25° C., more preferably less than or equal to 2 000 mPa·s at 25° C., and most preferably less than or equal to 1 500 mPa·s at 25° C.

According to one embodiment, the process further comprises step vi) of separating the precipitated calcium carbonate from the aqueous suspension obtained in step iv), and optionally step vii) of drying the separated precipitated calcium carbonate obtained in step vi).

According to another embodiment the process further comprises step vi) of separating the precipitated calcium carbonate from the aqueous suspension obtained in step iv), and optionally step vii) of drying the separated precipitated calcium carbonate obtained in step vi).

According to yet another embodiment, the process further comprises a step viii) of contacting at least a part of the surface of the obtained precipitated calcium carbonate with at least one hydrophobising agent after step iv) and/or after step vi), if present, and/or during and/or after step vii), if present, preferably the at least one hydrophobising agent is selected from the group consisting of an aliphatic carboxylic acid having a total amount of carbon atoms from C₄ to C₂₄ and/or reaction products thereof, a mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C₂ to C₃₀ in the substituent and/or reaction products thereof, a phosphoric acid ester blend of one or more phosphoric acid mono-ester and/or reaction products thereof and one or more phosphoric acid di-ester and/or reaction products thereof, polyhydrogensiloxane and reaction products thereof, an inert silicone oil, preferably polydimethylsiloxane, an aliphatic aldehyde having from 6 to 14 carbon atoms and/or reaction products thereof, and mixtures thereof.

It should be understood that for the purpose of the present invention, the following terms have the following meaning:

A “calcium oxide containing material” in the meaning of the present invention can be a mineral or a synthetic material having a content of calcium oxide of at least 50 wt.-%, preferably 75 wt.-%, more preferably 90 wt.-%, and most preferably 95 wt.-%, based on the total weight of the calcium oxide containing material. For the purpose of the present invention, a “mineral material” is a solid substance having a definite inorganic chemical composition and characteristic crystalline and/or amorphous structure.

“Ground calcium carbonate” (GCC) in the meaning of the present invention is a calcium carbonate obtained from natural sources, such as limestone, marble, or chalk, and processed through a wet and/or dry treatment such as grinding, screening and/or fractionation, for example by a cyclone or classifier.

Throughout the present document, the “particle size” of precipitated calcium carbonate or other particulate materials is described by its distribution of particle sizes. The value d_(x) represents the diameter relative to which x % by weight of the particles have diameters less than d_(x). This means that the d₂₀ value is the particle size at which 20 wt.-% of all particles are smaller, and the d₉₈ value is the particle size at which 98 wt.-% of all particles are smaller. The d₉₈ value is also designated as “top cut”. The d₅₀ value is thus the weight median particle size, i.e. 50 wt.-% of all grains are smaller than this particle size. For the purpose of the present invention the particle size is specified as weight median particle size d₅₀ unless indicated otherwise. For determining the weight median particle size d₅₀ value or the top cut particle size d₉₈ value a Sedigraph 5100 or 5120 device from the company Micromeritics, USA, can be used.

“Precipitated calcium carbonate” (PCC) in the meaning of the present invention is a synthesized material, generally obtained by precipitation following a reaction of carbon dioxide and calcium hydroxide (hydrated lime) in an aqueous environment or by precipitation of a calcium- and a carbonate source in water. Additionally, precipitated calcium carbonate can also be the product of introducing calcium and carbonate salts, calcium chloride and sodium carbonate for example, in an aqueous environment. PCC may have a vateritic, calcitic or aragonitic crystalline form. PCCs are described, for example, in EP 2 447 213 A1, EP 2 524 898 A1, EP 2 371 766 A1, or WO 2013/142473 A1.

A “suspension” or “slurry” in the meaning of the present invention comprises insoluble solids and water, and optionally further additives, and usually contains large amounts of solids and, thus, is more viscous and can be of higher density than the liquid from which it is formed.

For the purpose of the present invention, the “solids content” of a liquid composition such as a suspension or slurry is a measure of the amount of material remaining after all the solvent or water has been evaporated.

A “copolymer” in the meaning of the present invention is a polymer that has been prepared by polymerizing two or more different monomers together. The two or more different monomers of the copolymer of the present invention can be present in blocks, alternating or randomly.

A “specific BET surface area” (SSA) in the meaning of the present invention is defined as the surface area of the precipitated calcium carbonate particles divided by the mass of PCC particles. As used therein the specific surface area is measured by adsorption using the BET isotherm (ISO 9277:1995) and is specified in m²/g.

For the purpose of the present invention, the term “viscosity” or “Brookfield viscosity” refers to Brookfield viscosity. The Brookfield viscosity is for this purpose measured by a Brookfield DV-II+ Pro viscometer at 25° C.±1° C. at 100 rpm using an appropriate spindle of the Brookfield RV-spindle set and is specified in mPa·s. Based on his technical knowledge, the skilled person will select a spindle from the Brookfield RV-spindle set which is suitable for the viscosity range to be measured. For example, for a viscosity range between 200 and 800 mPa·s the spindle number 3 may be used, for a viscosity range between 400 and 1 600 mPa·s the spindle number 4 may be used, for a viscosity range between 800 and 3 200 mPa·s the spindle number 5 may be used, for a viscosity range between 1 000 and 2 000 000 mPa·s the spindle number 6 may be used, and for a viscosity range between 4 000 and 8 000 000 mPa·s the spindle number 7 may be used.

Unless specified otherwise, the term “drying” refers to a process according to which at least a portion of water is removed from a material to be dried such that a constant weight of the obtained “dried” material at 120° C. is reached. Moreover, a “dried” material may be further defined by its total moisture content which, unless specified otherwise, is less than or equal to 1.0 wt.-%, preferably less than or equal to 0.5 wt.-%, more preferably less than or equal to 0.2 wt.-%, and most preferably between 0.03 and 0.07 wt.-%, based on the total weight of the dried material.

The “total moisture content” of a material refers to the percentage of moisture (i.e. water) which may be desorbed from a sample upon heating to 220° C.

In the meaning of the present invention, “stable in an aqueous suspension having a pH of 12 and a temperature of 95° C.” means that the copolymer maintains its physical properties and chemical structure when added to an aqueous suspension having a pH of 12 and a temperature of 95° C. For example, the copolymer maintains its dispersing qualities and is not degraded under said conditions.

Where the term “comprising” is used in the present description and claims, it does not exclude other non-specified elements of major or minor functional importance. For the purposes of the present invention, the term “consisting of” is considered to be a preferred embodiment of the term “comprising of”. If hereinafter a group is defined to comprise at least a certain number of embodiments, this is also to be understood to disclose a group, which preferably consists only of these embodiments.

Whenever the terms “including” or “having” are used, these terms are meant to be equivalent to “comprising” as defined above.

Where an indefinite or definite article is used when referring to a singular noun, e.g. “a”, “an” or “the”, this includes a plural of that noun unless something else is specifically stated.

Terms like “obtainable” or “definable” and “obtained” or “defined” are used interchangeably. This e.g. means that, unless the context clearly dictates otherwise, the term “obtained” does not mean to indicate that e.g. an embodiment must be obtained by e.g. the sequence of steps following the term “obtained” even though such a limited understanding is always included by the terms “obtained” or “defined” as a preferred embodiment.

The inventive process for producing an aqueous suspension of precipitated calcium carbonate comprising the steps of i) providing a calcium oxide containing material, ii) providing at least one copolymer of the following formula (I)

wherein x, y and z are present in blocks, alternating or randomly; x is >0 and at least one of y or z is >0 and the sum of x+y+z is ≤150; R₁ represents hydrogen or a sulphonic functional group; R₂ represents a heteroatom, optionally substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group; R₃ and R₄ are independently from each other a hydroxyl group, (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation, an O-alkyl group comprising from 1 to 20 carbon atoms, an N-alkyl group comprising from 1 to 20 carbon atoms and/or a polyalkoxylated group; iii) preparing a milk of lime by mixing water, the calcium oxide containing material of step i), and the at least one copolymer of step ii) to obtain a milk of lime, wherein the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:12, and iv) carbonating the milk of lime obtained in step iii) to form an aqueous suspension of precipitated calcium carbonate.

In the following details and preferred embodiments of the inventive process will be set out in more details. It is to be understood that these technical details and embodiments also apply to the inventive products as well as to their use.

Process Step i): Provision of a Calcium Oxide Containing Material

According to step i) of the process of the present invention, a calcium oxide containing material is provided.

The calcium oxide containing material of step i) can be obtained by calcining a calcium carbonate containing material. Calcination is a thermal treatment process applied to calcium carbonate containing materials in order to bring about a thermal decomposition resulting in the formation of calcium oxide and gaseous carbon dioxide. Calcium carbonate containing materials which may be used in such a calcinations process are those selected from the group comprising precipitated calcium carbonates, natural calcium carbonate containing minerals such as marble, limestone and chalk, and mixed alkaline earth carbonate minerals comprising calcium carbonate such as dolomite, or calcium carbonate rich fractions from other sources. It is also possible to subject a calcium carbonate containing waste material to a calcinations process in order to obtain a calcium oxide containing material. Calcium carbonate decomposes at about 1 000° C. to calcium oxide (commonly known as quicklime). The calcination step may be carried out under conditions and using equipment well-known to the person skilled in the art. Generally, calcination may be carried out in furnaces or reactors (sometimes referred to as kilns) of various designs including shaft furnaces, rotary kilns, multiple hearth furnaces, and fluidized bed reactors.

The end of the calcination reaction may be determined, e.g. by monitoring the density change, the residual carbonate content, e.g. by X-ray diffraction, or the slaking reactivity by common methods.

According to one embodiment of the present invention, the calcium oxide containing material of step i) is obtained by calcining a calcium carbonate containing material, preferably selected from the group consisting of precipitated calcium carbonate, natural calcium carbonate minerals such as marble, limestone and chalk, mixed alkaline earth carbonate minerals comprising calcium carbonate such as dolomite, and mixtures thereof.

For reasons of efficiency, it is preferred that the calcium oxide containing material has a minimum calcium oxide content of at least 75 wt.-%, preferably at least 90 wt.-%, and most preferably 95 wt.-%, based on the total weight of the calcium oxide containing material. According to one embodiment, the calcium oxide containing material consists of calcium oxide.

The calcium oxide containing material can consist of only one type of calcium oxide containing material. Alternatively, the calcium oxide containing material can consist of a mixture of two or more types of calcium oxide containing materials.

The calcium oxide containing material can be used in the inventive process in its original form, i.e. as a raw material, for example, in form of smaller and bigger chunks. Alternatively, the calcium oxide containing material can be ground before use. According to one embodiment of the present invention, the calcium carbonate containing material is in forms of particles having weight median particle size d₅₀ from 0.1 to 1 000 μm, and preferably from 1 to 500 μm.

Process Step ii): Provision of at Least One Copolymer

According to step ii) of the process of the present invention, at least one copolymer is provided.

The term “at least one” copolymer in the meaning of the present invention means that the copolymer comprises, preferably consists of, one or more copolymer(s).

In one embodiment of the present invention, the at least one copolymer comprises, preferably consists of, one copolymer. Alternatively, the at least one copolymer comprises, preferably consists of, two or more copolymers. For example, the at least one copolymer comprises, preferably consists of, two copolymers.

Preferably, the at least one copolymer comprises, more preferably consists of, one copolymer.

It is one requirement of the present invention that the at least one copolymer is of the following formula (I)

wherein x, y and z are present in blocks, alternating or randomly; x is >0 and at least one of y or z is >0 and the sum of x+y+z is ≤150; R₁ represents hydrogen or a sulphonic functional group; R₂ represents a heteroatom, optionally substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group; R₃ and R₄ are independently from each other a hydroxyl group, (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation, an O-alkyl group comprising from 1 to 20 carbon atoms, an N-alkyl group comprising from 1 to 20 carbon atoms and/or a polyalkoxylated group.

As used herein, the term “alkyl” is a radical of a saturated aliphatic group, including linear chain alkyl groups, branched chain alkyl groups and cyclic chain alkyl groups comprising from 1 to 20 carbon atoms, wherein such linear, branched and cyclic chain alkyl groups may each be optionally substituted, e.g. with a hydroxyl group.

As used herein, the term “alkenyl” is a radical of an unsaturated aliphatic group, including linear chain alkenyl groups, branched chain alkenyl groups and cyclic chain alkenyl groups comprising from 2 to 20 carbon atoms, wherein such linear, branched and cyclic chain alkenyl groups may each be optionally substituted, e.g. with a hydroxyl group. The alkenyl group can feature one or more insaturations.

As used herein, the term “heteroatom” refers to an oxygen, sulphur or nitrogen atom, preferably an oxygen atom.

As used herein, the “sulphonic” functional group refers to a —SO₃H or (SO₃ ⁻, M⁺).

The term “polyalkoxylated group” in the meaning of the present invention refers to the group [(EO)_(n)(PO)_(n′)(BO)_(n″)]—Z. The alkoxylated units are present in blocks, alternating or randomly, and are selected from ethoxylated units EO, propoxylated units PO and butoxylated units BO. n, n′, n″ are independently from each other each 0 or an integer in the range from 1 to 150, the sum of n, n′ and n″ is >0 and Z is an alkyl group comprising from 1 to 20 carbon atoms, e.g. 1 or 2 carbon atoms. In one embodiment, R₂ is a nitrogen atom being substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group. For example, the nitrogen atom can be substituted with an alkyl group comprising a primary ammonium, secondary ammonium or tertiary ammonium function. Preferably, R₂ is N—CH₂—CH₂—N(CH₃)₂.

In the at least one copolymer of the formula (I), x, y and z can be present in blocks, alternating or randomly. Preferably, x, y and z are present alternating or randomly in the at least one copolymer of the formula (I). More preferably, x, y and z are present alternating in the at least one copolymer of the formula (I).

It is appreciated that x is >0 and at least one of y or z is >0 and the sum of x+y+z is ≤150 in the at least one copolymer of the formula (I).

In one embodiment, x, y and z are >0 and the sum of x+y+z is ≤150 in the at least one copolymer of the formula (I). For example, x, y and z are independently from each other each an integer in the range from 1 to 148 and the sum of x+y+z is an integer in the range from 3 to 150 in the at least one copolymer of the formula (I).

Thus, the at least one copolymer can be a compound of the formula (I), wherein x, y and z are present alternating or randomly; x, y and z are >0 and the sum of x+y+z is ≤150; R₁ represents hydrogen; R₂ represents a heteroatom, optionally substituted with an alkyl group; R₃ and R₄ are independently from each other a hydroxyl group, (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation, or an O-alkyl group comprising from 1 to 20 carbon atoms.

For example, the at least one copolymer can be a compound of the formula (I), wherein x, y and z are present alternating; x, y and z are independently from each other an integer in the range from 1 to 148 and the sum of x+y+z is an integer in the range from 3 to 150; R₁ represents hydrogen; R₂ represents a heteroatom; R₃ and R₄ are independently from each other a (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation.

Preferably, the at least one copolymer can be a compound of the formula (I), wherein x, y and z are present alternating; x, y and z are independently from each other an integer in the range from 1 to 148 and the sum of x+y+z is an integer in the range from 3 to 150; R₁ represents hydrogen; R₂ represents a heteroatom such as oxygen; R₃ and R₄ are independently from each other a (O⁻M⁺) with M⁺ being a monovalent cation such as sodium.

In another embodiment, it is preferred that x is >0 and y or z is 0.

For example, x and y are >0 and z is 0 and the sum of x+y+z is ≤150 in the at least one copolymer of the formula (I). For example, x and y are independently from each other each an integer in the range from 1 to 149 and z is 0 and the sum of x+y+z is an integer in the range from 2 to 150 in the at least one copolymer of the formula (I).

Alternatively, x and z are >0 and y is 0 and the sum of x+y+z is ≤150 in the at least one copolymer of the formula (I). For example, x and z are independently from each other each an integer in the range from 1 to 149 and y is 0 and the sum of x+y+z is an integer in the range from 2 to 150 in the at least one copolymer of the formula (I).

Thus, the at least one copolymer is preferably a compound of the following formula (II)

wherein x and y are present in blocks, alternating or randomly; x and y are >0 and the sum of x+y is ≤150; R₁ represents hydrogen or a sulphonic functional group; R₂ represents a heteroatom, optionally substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group.

For example, the at least one copolymer is a compound of the formula (II), wherein x and y are present alternating or randomly; x and y are independently from each other each an integer in the range from 1 to 149 and the sum of x+y is an integer in the range from 2 to 150; R₁ represents hydrogen or a sulphonic functional group; R₂ represents a heteroatom, optionally substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group.

Preferably, the at least one copolymer is a compound of the formula (II), wherein x and y are present alternating or randomly; x and y are independently from each other each an integer in the range from 1 to 149 and the sum of x+y is an integer in the range from 2 to 150; R₁ represents hydrogen; R₂ represents a heteroatom, optionally substituted with an alkyl group.

More preferably, the at least one copolymer is a compound of the formula (II), wherein x and y are present alternating; x and y are independently from each other each an integer in the range from 1 to 149 and the sum of x+y is an integer in the range from 2 to 150; R₁ represents hydrogen; R₂ represents a heteroatom such as oxygen.

Alternatively, the at least one copolymer is a compound of the following formula (III)

wherein x and z are present in blocks, alternating or randomly; x and z are >0 and the sum of x+z is ≤150; R₁ represents hydrogen or a sulphonic functional group; R₃ is a hydroxyl group, (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation, an O-alkyl group comprising from 1 to 20 carbon atoms, an N-alkyl group comprising from 1 to 20 carbon atoms and/or a polyalkoxylated group; and M⁺ being a monovalent, divalent or trivalent cation.

For example, the at least one copolymer is a compound of the formula (III), wherein x and z are present alternating or randomly; x and z are independently from each other each an integer in the range from 1 to 149 and the sum of x+z is an integer in the range from 2 to 150; R₁ represents hydrogen; R₃ is (O⁻M⁺) with M⁺ being a monovalent cation, an O-alkyl group comprising from 1 to 20 carbon atoms and/or a polyalkoxylated group; and M⁺ being a monovalent cation.

Preferably, the at least one copolymer is a compound of the formula (III), wherein x and z are present alternating or randomly; x and z are independently from each other each an integer in the range from 1 to 149 and the sum of x+z is an integer in the range from 2 to 150; R₁ represents hydrogen; R₃ is (O⁻M⁺) with M⁺ being a monovalent cation or an O-alkyl group comprising from 1 to 20 carbon atoms; and M⁺ being a monovalent cation.

More preferably, the at least one copolymer is a compound of the formula (III), wherein x and z are present alternating; x and z are independently from each other each an integer in the range from 1 to 149 and the sum of x+z is an integer in the range from 2 to 150; R₁ represents hydrogen; R₃ is (O⁻M⁺) with M⁺ being a monovalent cation; and M⁺ being a monovalent cation.

In one embodiment, the at least one copolymer is a compound of the formula (III), wherein x and z are present randomly; x and z are independently from each other each an integer in the range from 1 to 149 and the sum of x+z is an integer in the range from 2 to 150; R₁ represents hydrogen; R₃ is (O⁻M⁺) with M⁺ being a monovalent cation; and M⁺ being a monovalent cation, and M⁺ of (O⁻M⁺) and M⁺ being the same monovalent cation.

It is appreciated that the copolymer of step ii) can be partially or completely neutralized.

As regards the term “monovalent cation”, it is appreciated that this term refers to a cation having a valency of one. Accordingly, the term “divalent cation” refers to a cation having a valency of two and the term “trivalent cation” refers to a cation having a valency of three.

The monovalent cation is preferably selected from lithium, sodium, potassium and/or ammonium. More preferably, the monovalent cation is selected from lithium, sodium and/or potassium. Most preferably the monovalent cation is sodium and/or potassium. For example, the monovalent cation is sodium.

If the monovalent cation is ammonium, the at least one copolymer of step ii) is preferably partially neutralized. If the monovalent cation is lithium, sodium or potassium, preferably sodium, the at least one copolymer of step ii) is preferably completely neutralized.

In view of this, the at least one copolymer is preferably a compound of the formula (III), wherein x and z are present alternating; x and z are independently from each other each an integer in the range from 1 to 149 and the sum of x+z is an integer in the range from 2 to 150; R₁ represents hydrogen; R₃ is (O⁻M⁺) with M⁺ being sodium or ammonium, preferably sodium; and M⁺ being sodium or ammonium, preferably sodium.

The divalent cation is preferably selected from calcium and/or magnesium. More preferably, the divalent cation is calcium.

In this embodiment, the at least one copolymer is preferably a compound of the formula (III), wherein x and z are present alternating; x and z are independently from each other each an integer in the range from 1 to 149 and the sum of x+z is an integer in the range from 2 to 150; R₁ represents hydrogen; R₃ is (O⁻M⁺) with M⁺ being calcium; and M⁺ being calcium.

The trivalent cation is preferably selected from Al³⁺ and/or Fe³⁺.

It is appreciated that the at least one copolymer of step ii) is preferably obtained by polymerizing monomers selected from low molecular weight styrene and maleic anhydride. For example, the at least one copolymer of step ii) is a low molecular weight copolymer comprising, preferably consisting of, styrene and maleic anhydride, and/or derivatives thereof.

In one embodiment, the copolymer derivative, preferably the derivative of a copolymer comprising, preferably consisting of, styrene and maleic anhydride, comprises, preferably consists of:

-   -   units of maleic anhydride being partially or completely         hydrolyzed, and/or     -   units of maleic anhydride being partially or completely         esterified, and/or     -   units of maleic anhydride being partially or completely         amidified, and/or     -   units of maleic anhydride being partially or completely         imidized, and/or     -   units of styrene being partially or completely sulfonized.

In order to obtain a PCC suspension with a high solids content at an acceptable viscosity, it is especially preferred that the monomers/comonomers of the at least one copolymer of step ii) are present in a specific molar ratio.

In one embodiment, the molar ratio of x to y and/or z in the at least one copolymer of step ii) [x:y and/or z] is from 10:1 to 1:2, preferably from 5:1 to 1:2 and most preferably about 1:1, 2:1 or 3:1.

Preferably, the molar ratio of x to y and z in the at least one copolymer of step ii) [x:y and z] is from 10:1 to 1:2, preferably from 5:1 to 1:2 and most preferably about 1:1, 2:1 or 3:1. Alternatively, the molar ratio of x to y or z in the at least one copolymer of step ii) [x:y or z] is from 10:1 to 1:2, preferably from 5:1 to 1:2 and most preferably about 1:1, 2:1 or 3:1. For example, the molar ratio of x to z in the at least one copolymer of step ii) [x: z] is from 10:1 to 1:2, preferably from 5:1 to 1:2 and most preferably about 1:1, 2:1 or 3:1. This embodiment especially applies, in case the at least one copolymer of step ii) is a compound of the formula (III).

According to one embodiment of the present invention, the at least one copolymer of step ii) has a molecular weight M_(w) in the range from 500 to 100 000 g/mol, preferably from 1 000 to 50 000 g/mol, more preferably from 2 000 to 20 000 g/mol and most preferably from 2 500 to 10 000 g/mol. The molecular weight M_(w) may be determined by gel permeation chromatography.

Additionally or alternatively, the at least one copolymer of step ii), provided in a water solution, has a Brookfield viscosity of less than or equal to 5 000 mPa·s at 25° C., more preferably less than or equal to 2 000 mPa·s at 25° C., and most preferably less than or equal to 1 500 mPa·s at 25° C. Preferably, the at least one copolymer of step ii), provided in a water solution, has a Brookfield viscosity of less than or equal to 5 000 mPa·s at 25° C., more preferably less than or equal to 2 000 mPa·s at 25° C., and most preferably less than or equal to 1 500 mPa·s at 25° C., at copolymer content of 20 to 40 wt.-%, based on the total weight of the copolymer solution.

More preferably, the at least one copolymer of step ii), provided in a water solution, has a Brookfield viscosity of less than or equal to 1 000 mPa·s at 25° C., and most preferably in the range from 100 to 1 000 mPa·s at 25° C. Preferably, the at least one copolymer of step ii), provided in a water solution, has a Brookfield viscosity of less than or equal to 1 000 mPa·s at 25° C., and most preferably in the range from 100 to 1 000 mPa·s at 25° C., at copolymer content of 20 to 40 wt.-%, based on the total weight of the copolymer solution.

According to one embodiment of the present invention, the at least one copolymer of step ii) is added in an amount from 0.001 to 5 wt.-%, preferably from 0.01 to 2 wt.-%, more preferably from 0.05 to 1 wt.-%, and most preferably from 0.1 to 0.5 wt.-%, based on the total weight of the calcium oxide containing material in the milk of lime.

The at least one copolymer of step ii) can be provided in form of a solution or as a dry material. According to one embodiment, the at least one copolymer of step ii) is provided in form of a solution. According to another embodiment of the present invention, the at least one copolymer of step ii) is provided in form of an aqueous solution having a polymer concentration from 1 to 70 wt.-%, and preferably from 2 to 60 wt.-%, based on the total weight of the aqueous solution.

According to the present invention, the at least one copolymer is added during step iii) of the inventive process for producing PCC, i.e. the copolymer is added during the slaking step. As known to the skilled person, the milk of lime obtained by slaking a calcium oxide containing material with water has usually a pH value between 11 and 12.5 at a temperature of 25° C., depending on the concentration of the calcium oxide containing material in the milk of lime. Since the slaking reaction is exothermic, the temperature of the milk of lime typically raises to a temperature between 80 and 99° C. According to one embodiment of the present invention, the at least one copolymer of step ii) is selected such that it is stable in an aqueous suspension having a pH of 12 and a temperature of 95° C. In the meaning of the present invention, “stable in an aqueous suspension having a pH of 12 and a temperature of 95° C.” means that the copolymer maintains its physical properties and chemical structure when added to an aqueous suspension having a pH of 12 and a temperature of 95° C. For example, the copolymer maintains its dispersing qualities and is not degraded under said conditions.

Process Step iii): Preparing of the Milk of Lime

According to step iii) of the process of the present invention, a milk of lime is prepared by mixing water, the calcium oxide containing material of step i), and the at least one copolymer of step ii) to obtain a milk of lime, wherein the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:12.

The reaction of the calcium oxide containing material with water results in the formation of a milky calcium hydroxide suspension, better known as milk of lime. Said reaction is highly exothermic and is also designated as “lime slaking” in the art.

According to one embodiment of the present invention, the temperature of the water, which is used in mixing step iii), i.e. the temperature of the water that is used to slake the calcium oxide containing material, is adjusted to be in the range from more than 0° C. and less than 100° C. In other words, the water that is used to slake the calcium oxide containing material is adjusted to a temperature range, in which the water is in liquid form. Preferably, the temperature of the water, which is employed in mixing step iii) is adjusted to be from 1° C. to 85° C., more preferably from 2° C. to 70° C., even more preferably from 30° C. to 65° C., and most preferably from 35 to 55° C. It will be apparent to the skilled person that the initial temperature of the water is not necessarily the same one as the temperature of the mixture prepared in step iii) due to the highly exothermic slaking reaction and/or due to the mixing of substances having different temperatures.

According to one embodiment of the present invention, process step iii) comprises the steps of:

-   -   a1) mixing the at least one copolymer of step ii) with water,         and     -   a2) adding the calcium oxide containing material of step i) to         the mixture of step a1).

According to one embodiment, step a1) is carried out at a temperature from more than 0° C. to 99° C., preferably from 1° C. to 75° C., more preferably from 2° C. to 70° C., even more preferably from 30° C. to 65° C., and most preferably from 35 to 55° C.

According to another embodiment of the present invention, process step iii) comprises the steps of:

-   -   b1) mixing the calcium oxide containing material of step i), and         the at least one copolymer of step ii), and     -   b2) adding water to the mixture of step b1).

According to still another embodiment of the present invention, in process step iii) the calcium oxide containing material of step i), the at least one copolymer of step ii), and water are mixed simultaneously.

The at least one copolymer of step ii) may be added in step iii) in one portion or in several portions. According to one embodiment, in step iii) the at least one copolymer of step ii) is mixed with the water, and the calcium oxide containing material of step i), by adding the at least one copolymer in one portion or in two, three, four, five, or more portions.

Process step iii) may be performed at room temperature, i.e. at a temperature of 20° C.±2° C., or at an initial temperature of 30 to 50°, preferably 35 to 45° C. Since the reaction is exothermic, the temperature typically raises to a temperature between 85 and 99° C. during step iii), preferably to a temperature between 90 and 95° C. According to a preferred embodiment, process step iii) is performed under mixing, agitation, or stirring, for example, mechanical stirring. Suitable process equipment for mixing, agitation or stirring is known to the skilled person.

The progress of the slaking reaction may be observed by measuring the temperature and/or conductivity of the reaction mixture. It can also be monitored by turbidity control. Alternatively or additionally, the progress of the slaking reaction can be inspected visually.

Conventional methods for preparing PCC suffer from the problem that the milk of lime can only be processed at low solids content since the milk of lime becomes very viscous at higher solids content during the slaking process. In a typical PCC production process of the prior art, the weight ratio of calcium oxide to water is less than 1:6, usually 1:9 or 1:10. The inventors surprisingly found that the addition of a copolymer as defined above, before or during the slaking step of a process for producing PCC can allow the preparation of a milk of lime with a high solids content. By carbonating said highly concentrated milk of lime, an aqueous suspension of PCC can be obtained which has also a high solids content. As a result, the process of the present invention does not require an additional concentration step in order to obtain a PCC suspension with high solids content.

According to the present invention, the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:12. According to one preferred embodiment, in step iii) the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:9, more preferably from 1:2.5 to 1:5.

According to one embodiment of the present invention, the milk of lime of step iii) has a solids content of at least 8 wt.-%, preferably from 10 to 66 wt.-%, more preferably from 12 to 66 wt.-%, even more preferably from 15 to 55 wt.-%, and most preferably from 17 to 45 wt.-%, such as from 20 to 38 wt.-%, based on the total weight of the milk of lime.

According to one embodiment of the present invention, the milk of lime obtained in step iii) has a Brookfield viscosity from 1 to 1 000 mPa·s at 25° C., more preferably from 5 and 800 mPa·s at 25° C., and most preferably from 10 and 500 mPa·s at 25° C. According to one embodiment, the Brookfield viscosity is measured at 100 rpm. Preferably, the milk of lime obtained in step iii) has a Brookfield viscosity from 1 to 1 000 mPa·s at 25° C., more preferably from 5 and 800 mPa·s at 25° C., and most preferably from 10 and 500 mPa·s at 25° C., at solids content of at least 8 wt.-%, preferably from 10 to 66 wt.-%, more preferably from 12 to 66 wt.-%, even more preferably from 15 to 55 wt.-%, and most preferably from 17 to 45 wt.-%, such as from 20 to 38 wt.-%, based on the total weight of the milk of lime.

It is within the confines of the present invention that additional water may be introduced during the slaking reaction in order to control and/or maintain and/or achieve the desired solids content or Brookfield viscosity of the milk of lime.

Process step iii) can be carried out in form of a batch process, a semi-continuous or a continuous process. FIG. 1 shows an example of a continuous process step iii). The at least on copolymer (2), the optional slaking additive (3), water (4), and a calcium oxide containing material (5) are fed into a slaker (1). The reaction heat (6) resulting from the exothermic slaking reaction is dissipated and the obtained milk of lime is discharged (7) to the next process stage, for example, the carbonation stage or a screening stage.

Process Step iv): Carbonating the Milk of Lime

According to step iv) of the process of the present invention, the milk of lime obtained from step iii) is carbonated to form an aqueous suspension of precipitated calcium carbonate.

The carbonation is carried out by means and under conditions well-known by the person skilled in the art. The introduction of carbon dioxide into the milk of lime quickly increases the carbonate ion (CO₃ ²⁻) concentration and calcium carbonate is formed. Particularly, the carbonation reaction can be readily controlled considering the reactions involved in the carbonation process. Carbon dioxide dissolves according to its partial pressure forming carbonate ions via the formation of carbonic acid (H₂CO₃), and hydrogen carbonate ions (HCO₃ ⁻) being unstable in the alkaline solution. Upon continued dissolution of carbon dioxide, hydroxide ions are consumed and the concentration of carbonate ions increases until the concentration of dissolved calcium carbonate exceeds the solubility product and solid calcium carbonate precipitates.

According to one embodiment of the present invention, in step iv) the carbonation is carried out by feeding pure gaseous carbon dioxide or technical gases containing at least 10 vol.-% of carbon dioxide into the milk of lime.

The progress of the carbonation reaction can be readily observed by measuring the conductivity density, turbidity and/or pH. In this respect, the pH of the milk of lime before addition of carbon dioxide will be more than 10, usually between 11 and 12.5, and will constantly decrease until a pH of about 7 is reached. At this point the reaction can be stopped.

Conductivity slowly decreases during the carbonation reaction and rapidly decreases to low levels, when the precipitation is completed. The progress of the carbonation may be monitored by measuring the pH and/or the conductivity of the reaction mixture.

According to one embodiment of the present invention, the temperature of the milk of lime obtained from step iii), which is used in step iv) is adjusted to be in the range from 20° C. to 65° C., and preferably from 30° C. to 55° C. It will be apparent to the skilled person that the initial temperature of the milk of lime, is not necessarily the same one as the temperature of the mixture prepared in step iv) due to the exothermic carbonation reaction and/or due to the mixing of substances having different temperatures.

According to one embodiment of the present invention, step iv) is carried out at a temperature from 5 to 95° C., preferably from 30 to 70° C., and more preferably from 40 to 60° C.

Process step iv) can be carried out in form of a batch process, a semi-continuous or a continuous process. According to one embodiment, the process of the present invention involving the process steps i) to iv) is carried out in form of a batch process, a semi-continuous or a continuous process.

According to one embodiment of the present invention, the process of the present invention does not comprise a step of up-concentrating the aqueous suspension of precipitated calcium carbonate obtained by steps i) to iv) of the inventive process. Additionally or alternatively, the process of the present invention does not comprise a step of separating the liquid phase from the solids content in the suspension obtained in step iii), i.e. there is no step of separating carried out between steps iii) and iv) of the inventive process.

As already mentioned above, the inventors surprisingly found that the addition of at least one copolymer as defined above before or during the slaking step of a process for producing PCC can allow the preparation of a PCC suspension with a high solids content. It is also believed that the omission of a concentration step improves the quality of the produced PCC particles, since surface damages of the particles, which can occur during the concentration step, are avoided. It was also found that said PCC suspension can be further up-concentrated to a solids contents of about 70 wt.-% at acceptable viscosities, for example, to Brookfield viscosities of less than or equal to 1 600 mPa·s at 25° C. and 100 rpm. Typically, this cannot be done with PCC suspensions that are obtained by conventional PCC production processes including an up-concentrating step because the viscosity of said suspension would rise to a non-pumpable range.

According to one embodiment of the present invention, the obtained precipitated calcium carbonate has a weight median particle size d₅₀ from 0.1 to 100 μm, preferably from 0.25 to 50 μm, more preferably from 0.3 to 5 μm, and most preferably from 0.4 to 3.0 μm.

The precipitated calcium carbonate may have aragonitic, calcitic, or vateritic crystal structure, or mixtures thereof. It is a further advantage of the present invention that the crystal structure and morphology of the precipitated calcium carbonate can be controlled, e.g. by addition of seed crystals or other structure modifying chemicals. According to a preferred embodiment, the precipitated calcium carbonate obtained by the inventive process has a clustered scalenohedral crystal structure.

The BET specific surface area of the precipitated calcium carbonate obtained by the process according to the present invention may be from 1 to 100 m²/g, preferably from 2 to 70 m²/g, more preferably from 3 to 50 m²/g, especially from 4 to 30 m²/g, measured using nitrogen and the BET method according to ISO 9277. The BET specific surface area of the precipitated calcium carbonate obtained by the process of the present invention may be controlled by the use of additives, e.g. surface active agents, shearing during the precipitation step or thereafter at high mechanical shearing rates not only leading to a low particle size, but also to a high BET specific surface area.

According to one embodiment of the present invention, the suspension of precipitated calcium carbonate obtained in step iv) has preferably a solids content of at least 15 wt.-%, preferably from 15 to 70 wt.-%, more preferably from 19 to 60 wt.-%, even more preferably from 21 to 50 wt.-% and most preferably from 24 to 42 wt.-%, based on the total weight of the suspension.

According to one embodiment of the present invention, the suspension of PCC obtained in step iv) has a Brookfield viscosity of less than or equal to 3 000 mPa·s at 25° C. For example, the suspension of PCC obtained in step iv) has a Brookfield viscosity of less than or equal to 3 000 mPa·s at 25° C., at solids content of at least 15 wt.-%, preferably from 15 to 70 wt.-%, more preferably from 19 to 60 wt.-%, even more preferably from 21 to 50 wt.-% and most preferably from 24 to 42 wt.-%, based on the total weight of the suspension.

Preferably, the suspension of PCC obtained in step iv) has a Brookfield viscosity of less than or equal to 2 500 mPa·s at 25° C., more preferably less than or equal to 2 000 mPa·s at 25° C., and most preferably less than or equal to 600 mPa·s at 25° C. The Brookfield viscosity is measured at 100 rpm. For example, the suspension of PCC obtained in step iv) has a Brookfield viscosity of less than or equal to 2 500 mPa·s at 25° C., more preferably less than or equal to 2 000 mPa·s at 25° C., and most preferably less than or equal to 600 mPa·s at 25° C., at solids content of at least 15 wt.-%, preferably from 15 to 70 wt.-%, more preferably from 19 to 60 wt.-%, even more preferably from 21 to 50 wt.-% and most preferably from 24 to 42 wt.-%, based on the total weight of the suspension.

Optional Process Steps

In one embodiment, the process further comprises step v) of adding at least one slaking additive to process step iii).

According to one embodiment of the present invention, the at least one slaking additive is selected from the group consisting of organic acids, organic acid salts, sugar alcohols, monosaccharides, disaccharides, polysaccharides, gluconates, phosphonates, lignosulphonates, and mixtures thereof.

For example, the at least one slaking additive is selected from the group consisting of sodium citrate, potassium citrate, calcium citrate, magnesium citrate, monosaccharides, disaccharides, polysaccharides, sucrose, sugar alcohols, meritol, citric acid, sorbitol, sodium salt of diethylene triamine pentaacetic acid, gluconates, phosphonates, sodium tartrate, sodium lignosulphonate, calcium lignosulphonate, and mixtures thereof. According to a preferred embodiment, the at least one slaking additive is sodium citrate and/or saccharose.

According to one embodiment of the present invention, the at least one slaking additive consists of one type of slaking additive only. Alternatively, the at least one slaking additive can consist of a mixture of two or more types of slaking additives.

The at least one slaking additive may be provided in an amount from 0.01 to 2.0 wt.-%, based on the total amount of calcium oxide containing material, preferably in an amount from 0.05 to 1.0 wt.-%, more preferably from 0.06 to 0.8 wt.-%, and most preferably from 0.07 to 0.5 wt.-%.

By adding a slaking additive, the size of the PCC particles and their crystal morphology can be controlled without affecting the viscosity of the final aqueous suspension.

If the process of the present invention comprises a step of adding at least one slaking additive to process step iii), the process step iii) preferably comprises the steps of:

-   -   a1) mixing the at least one copolymer of step ii) and the at         least one slaking additive with water, and     -   a2) adding the calcium oxide containing material of step i) to         the mixture of step a1).

Alternatively, process step iii) comprises the steps of:

-   -   b1) mixing the calcium oxide containing material of step i), the         at least one copolymer of step ii), and the at least one slaking         additive, and     -   b2) adding water to the mixture of step b1).

Alternatively, in process step iii) the calcium oxide containing material of step i), the at least one copolymer of step ii), the at least one slaking additive, and water are mixed simultaneously.

According to still another embodiment of the present invention, the at least one slaking additive is added before or after step iii) of the inventive process.

It is appreciated that the Brookfield viscosity of the suspension of PCC obtained in step iv) may vary if at least one slaking additive is added to process step iii).

For example, if at least one slaking additive is added to process step iii), the suspension of PCC obtained in step iv) preferably has a Brookfield viscosity of less than or equal to 2 000 mPa·s at 25° C., e.g. less than or equal to 1 500 mPa·s at 25° C. For example, the suspension of PCC obtained in step iv) has a Brookfield viscosity in the range from 10 to 1 500 mPa·s at 25° C. such as from 100 to 6 mPa·s at 25° C. Preferably, if at least one slaking additive is added to process step iii), the suspension of PCC obtained in step iv) has a Brookfield viscosity of less than or equal to 2 000 mPa·s at 25° C., more preferably less than or equal to 1 500 mPa·s at 25° C., even more preferably in the range from 10 to 1 500 mPa·s at 25° C. and most preferably from 100 to 600 mPa·s at 25° C., at solids content of at least 15 wt.-%, preferably from 15 to 70 wt.-%, more preferably from 19 to 60 wt.-%, even more preferably from 21 to 50 wt.-% and most preferably from 24 to 42 wt.-%, based on the total weight of the suspension.

If process step iii) is carried out in the absence of at least one slaking additive, i.e. no slaking additive is added to process step iii), the suspension of PCC obtained in step iv) preferably has a Brookfield viscosity of less than or equal to 3 000 mPa·s at 25° C., more preferably less than or equal to 2 500 mPa·s at 25° C., and most preferably less than or equal to 2 000 mPa·s at 25° C. Preferably, if process step iii) is carried out in the absence of at least one slaking additive, the suspension of PCC obtained in step iv) preferably has a Brookfield viscosity of less than or equal to 3 000 mPa·s at 25° C., more preferably less than or equal to 2 500 mPa·s at 25° C., and most preferably less than or equal to 2 000 mPa·s at 25° C., e.g. in the range from 1 000 to 2 000 mPa·s at 25° C. or from 1 500 to 2 000 mPa·s at 25° C., at solids content of at least 15 wt.-%, preferably from 15 to 70 wt.-%, more preferably from 19 to 60 wt.-%, even more preferably from 21 to 50 wt.-% and most preferably from 24 to 42 wt.-%, based on the total weight of the suspension.

In one embodiment of the present application, the milk of lime may be screened in order to remove oversize particles. A suitable screen can include, for example, a screen having a sieve size from 700 to 100 μm, for example, about 100 or about 300 μm. According to one embodiment of the present invention, the milk of lime is screened after step iii) and before step iv), preferably with a screen having a sieve size from 100 to 300 μm. It is to be noted that such a screening step is to be distinguished from a separating step as only particles of a specific size are removed. In contrast thereto, a separating step essentially completely removes the solids from an aqueous suspension.

It is possible to separate the precipitated calcium carbonate from the aqueous suspension obtained in step iv). In one embodiment, the process according to the present invention comprising the steps i) to iv), and optionally step v), thus further comprises step vi) of separating the precipitated calcium carbonate from the aqueous suspension obtained in step iv).

For the purpose of the present invention, the expression “separating” means that the PCC is removed or isolated from the aqueous suspension obtained from step iv) of the inventive process. The precipitated calcium carbonate obtained from step iv) may be separated from the mother liquor by any conventional means of separation known to the skilled person. According to one embodiment of the present invention, in process step vi) the PCC is separated mechanically and/or thermally. Examples for mechanical separation processes are filtration, e.g. by means of a drum filter or filter press, nanofiltration, or centrifugation. An example for a thermal separation process is a concentration process by the application of heat, for example, in an evaporator. According to a preferred embodiment, in process step vi) the PCC is separated mechanically, preferably by filtration and/or centrifugation.

It is also preferred that the mother liquor obtained after precipitation and/or any one of the reactants may be recycled into the process.

The obtained PCC may be further processed, e.g., may be deagglomerated or subjected to a dry grinding step. Otherwise, it may also be wet ground in form of a suspension. If the PCC is subjected to dewatering, dispersion and/or grinding steps, these steps may be accomplished by procedures known in the art. Wet grinding may be carried out in the absence of a grinding aid or in the presence of a grinding aid. One or more grinding agents can be included, such as, e.g., sodium polyacrylate, a salt of polyacrylate acid, and/or a salt of a copolymer of acrylic acid. Dispersants also can be included to prepare dispersions if desired.

In one embodiment, the separated precipitated calcium carbonate obtained from step vi) is dried in drying step vii). Thus, a process for producing a precipitated calcium carbonate is provided comprising the steps of:

-   -   i) providing a calcium oxide containing material,     -   ii) providing at least one copolymer of the following formula         (I)

-   -   wherein x, y and z are present in blocks, alternating or         randomly; x is >0 and at least one of y or z is >0 and the sum         of x+y+z is ≤150; R₁ represents hydrogen or a sulphonic         functional group; R₂ represents a heteroatom, optionally         substituted with an alkyl group, an alkenyl group, an heteroaryl         group and/or a polyalkoxylated group; R₃ and R₄ are         independently from each other a hydroxyl group, (O⁻M⁺) with M⁺         being a monovalent, divalent or trivalent cation, an O-alkyl         group comprising from 1 to 20 carbon atoms, an N-alkyl group         comprising from 1 to 20 carbon atoms and/or a polyalkoxylated         group     -   iii) preparing a milk of lime by mixing water, the calcium oxide         containing material of step i), and the at least one copolymer         of step ii) to obtain a milk of lime, wherein the calcium oxide         containing material and the water are mixed in a weight ratio         from 1:1 to 1:12,     -   iv) carbonating the milk of lime obtained in step iii) to form         an aqueous suspension of precipitated calcium carbonate,     -   v) optionally adding at least one slaking additive to process         step iii),     -   vi) separating the precipitated calcium carbonate from the         aqueous suspension obtained in step iv), and     -   vii) drying the precipitated calcium carbonate obtained in step         vi).

In general, the drying step vii) may take place using any suitable drying equipment and can, for example, include thermal drying and/or drying at reduced pressure using equipment such as an evaporator, a flash drier, an oven, a spray drier and/or drying in a vacuum chamber.

According to one embodiment, drying step vii) is a spray drying step, preferably said spray drying step is carried out at a lower temperature ranging from 90° C. to 130° C. and preferably from 100° C. to 120° C. By means of drying step vii), a dried precipitated calcium carbonate is obtained having a low total moisture content which is less than or equal to 1.0 wt.-%, based on the total weight of the dried precipitated calcium carbonate.

According to another embodiment, the dried PCC of step vii) has a total moisture content of less than or equal to 0.5 wt.-% and preferably less than or equal to 0.2 wt.-%, based on the total weight of the dried precipitated calcium carbonate. According to still another embodiment, the dried PCC of step vii) has a total moisture content of between 0.01 and 0.15 wt.-%, preferably between 0.02 and 0.10 wt.-%, and more preferably between 0.03 and 0.07 wt.-%, based on the total weight of the dried precipitated calcium carbonate.

The precipitated calcium carbonate obtained by the inventive process can be post-treated, for example, during and/or after a drying step with an additional component.

According to one embodiment the process of the present invention further comprises a step viii) of contacting at least a part of the surface of the obtained precipitated calcium carbonate with at least one hydrophobising agent after step iv) and/or after step vi), if present, and/or during and/or after step vii), if present. Preferably, at least a part of the surface of the obtained precipitated calcium carbonate may be contacted with at least one hydrophobising agent during and/or after step vii).

Suitable hydrophobising agents are, for example, aliphatic carboxylic acid having a total amount of carbon atoms from C₄ to C₂₄ and/or reaction products thereof, a mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C₂ to C₃₀ in the substituent and/or reaction products thereof, a phosphoric acid ester blend of one or more phosphoric acid mono-ester and/or reaction products thereof and one or more phosphoric acid di-ester and/or reaction products thereof, polyhydrogensiloxane and reaction products thereof, an inert silicone oil, preferably polydimethylsiloxane, an aliphatic aldehyde having from 6 to 14 carbon atoms and/or reaction products thereof, and mixtures thereof. Suitable hydrophobising agents and methods for preparing surface-treated filler products thereof are, for example, described in EP 2 159 258 A1, EP 2 390 285 A1, EP 2 390 280 A1, WO 2014/060286 A1, EO 2014/128087 A1, EP 2 722 368 A1 and EP 2 770 017 A1 which are thus incorporated herewith by references.

In one embodiment, the hydrophobising agent is an aliphatic carboxylic acid having a total amount of carbon atoms from C₄ to C₂₄ and/or reaction products thereof. The term “reaction products” of the aliphatic carboxylic acid in the meaning of the present invention refers to products obtained by contacting the modified mineral-based filler with the at least one aliphatic carboxylic acid. Said reaction products are formed between at least a part of the at least one aliphatic carboxylic acid and reactive molecules located at the surface of the alkaline earth metal carbonate-comprising material particles.

The aliphatic carboxylic acid in the meaning of the present invention may be selected from one or more straight chain, branched chain, saturated, unsaturated and/or alicyclic carboxylic acids. Preferably, the aliphatic carboxylic acid is a monocarboxylic acid, i.e. the aliphatic carboxylic acid is characterized in that a single carboxyl group is present. Said carboxyl group is placed at the end of the carbon skeleton.

In one embodiment of the present invention, the aliphatic carboxylic acid is selected from saturated unbranched carboxylic acids, that is to say the aliphatic carboxylic acid is preferably selected from the group of carboxylic acids consisting of pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, heptadecanoic acid, stearic acid, nonadecanoic acid, arachidic acid, heneicosylic acid, behenic acid, tricosylic acid, lignoceric acid and mixtures thereof.

In another embodiment of the present invention, the aliphatic carboxylic acid is selected from the group consisting of octanoic acid, decanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid and mixtures thereof. Preferably, the aliphatic carboxylic acid is selected from the group consisting of myristic acid, palmitic acid, stearic acid and mixtures thereof. For example, the aliphatic carboxylic acid is stearic acid.

Additionally or alternatively, the hydrophobising agent can be at least one mono-substituted succinic acid and/or salty reaction product(s) and/or at least one phosphoric acid ester blend of one or more phosphoric acid mono-ester and/or reaction products thereof and one or more phosphoric acid di-ester and/or reaction products thereof. Methods for treating a calcium carbonate-comprising material with these hydrophobising agents are described, for example, in EP 2 722 368 A1 and EP 2 770 017 A1 which are thus incorporated herewith by references.

The term “succinic anhydride”, also called dihydro-2,5-furandione, succinic acid anhydride or succinyl oxide, has the molecular formula C₄H₄O₃ and is the acid anhydride of succinic acid. The term “mono-substituted succinic anhydride” in the meaning of the present invention refers to a succinic anhydride wherein a hydrogen atom is substituted by another substituent.

The term “reaction products of at least one mono-substituted succinic anhydride” in the meaning of the present invention refers to products obtained by contacting a alkaline earth metal carbonate-comprising material with one or more mono-substituted succinic anhydride(s). Said salty reaction products are formed between the mono-substituted succinic acid which is formed from the applied mono-substituted succinic anhydride and reactive molecules located at the surface of the alkaline earth metal carbonate-comprising material.

The term “phosphoric acid mono-ester” in the meaning of the present invention refers to an o-phosphoric acid molecule mono-esterified with one alcohol molecule selected from unsaturated or saturated, branched or linear, aliphatic or aromatic alcohols having a total amount of carbon atoms from C₆ to C₃₀, preferably from C₈ to C₂₂, more preferably from C₈ to C₂₀, and most preferably from C₈ to C₁₈ in the alcohol substituent. The term “phosphoric acid di-ester” in the meaning of the present invention refers to an o-phosphoric acid molecule di-esterified with two alcohol molecules selected from the same or different, unsaturated or saturated, branched or linear, aliphatic or aromatic alcohols having a total amount of carbon atoms from C₆ to C₃₀, preferably from C₈ to C₂₂, more preferably from C₈ to C₂₀, and most preferably from C₈ to C₁₈ in the alcohol substituent.

The term “salty reaction products of a phosphoric acid ester or blend of one or more phosphoric acid mono-esters and/or one or more phosphoric acid di-esters” in the meaning of the present invention refers to products obtained by contacting an alkaline earth metal carbonate-comprising material with one or more phosphoric acid mono-ester and one or more phosphoric acid di-ester and optionally phosphoric acid. Said salty reaction products are formed between the applied one or more phosphoric acid mono-ester and one or more phosphoric acid di-ester and optionally phosphoric acid and reactive molecules located at the surface of the alkaline earth metal carbonate-comprising material.

According to one embodiment, the at least one hydrophobising agent is selected from the group consisting of an aliphatic carboxylic acid having a total amount of carbon atoms from C₄ to C₂₄ and/or reaction products thereof, a mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C₂ to C₃₀ in the substituent and/or reaction products thereof, a phosphoric acid ester blend of one or more phosphoric acid mono-ester and/or reaction products thereof and one or more phosphoric acid di-ester and/or reaction products thereof, polyhydrogensiloxane and reaction products thereof, an inert silicone oil, preferably polydimethylsiloxane, an aliphatic aldehyde having from 6 to 14 carbon atoms and/or reaction products thereof, and mixtures thereof.

According to a preferred embodiment, the at least one hydrophobising agent is a mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C₂ to C₃₀ in the substituent and/or a reaction product thereof

Products and Their Use

According to the present invention, an aqueous suspension of precipitated calcium carbonate is provided, which is obtainable by a process comprising the steps of:

-   -   i) providing a calcium oxide containing material,     -   ii) providing at least one copolymer of the following formula         (I)

-   -   wherein x, y and z are present in blocks, alternating or         randomly; x is >0 and at least one of y or z is >0 and the sum         of x+y+z is ≤150; R₁ represents hydrogen or a sulphonic         functional group; R₂ represents a heteroatom, optionally         substituted with an alkyl group, an alkenyl group, an heteroaryl         group and/or a polyalkoxylated group; R₃ and R₄ are         independently from each other a hydroxyl group, (O⁻M⁺) with M⁺         being a monovalent, divalent or trivalent cation, an O-alkyl         group comprising from 1 to 20 carbon atoms, an N-alkyl group         comprising from 1 to 20 carbon atoms and/or a polyalkoxylated         group,     -   iii) preparing a milk of lime by mixing water, the calcium oxide         containing material of step i), and the at least one copolymer         of step ii) to obtain a milk of lime, wherein the calcium oxide         containing material and the water are mixed in a weight ratio         from 1:1 to 1:12, and     -   iv) carbonating the milk of lime obtained from step iii) to form         an aqueous suspension of precipitated calcium carbonate, and     -   v) optionally, adding at least one slaking additive to process         step iii).

According to a further aspect of the present invention, a precipitated calcium carbonate is provided, which is obtainable by a process comprising the steps of:

-   -   i) providing a calcium oxide containing material,     -   ii) providing at least one copolymer of the following formula         (I)

-   -   wherein x, y and z are present in blocks, alternating or         randomly; x is >0 and at least one of y or z is >0 and the sum         of x+y+z is ≤150; R₁ represents hydrogen or a sulphonic         functional group; R₂ represents a heteroatom, optionally         substituted with an alkyl group, an alkenyl group, an heteroaryl         group and/or a polyalkoxylated group; R₃ and R₄ are         independently from each other a hydroxyl group, (O⁻M⁺) with M⁺         being a monovalent, divalent or trivalent cation, an O-alkyl         group comprising from 1 to 20 carbon atoms, an N-alkyl group         comprising from 1 to 20 carbon atoms and/or a polyalkoxylated         group,     -   iii) preparing a milk of lime by mixing water, the calcium oxide         containing material of step i), and the at least one copolymer         of step ii) to obtain a milk of lime, wherein the calcium oxide         containing material and the water are mixed in a weight ratio         from 1:1 to 1:12,     -   iv) carbonating the milk of lime obtained from step iii) to form         an aqueous suspension of precipitated calcium carbonate,     -   v) optionally, adding at least one slaking additive to process         step iii), and     -   vi) separating the precipitated calcium carbonate from the         aqueous suspension obtained from step iv).

Optionally, the obtained precipitated calcium carbonate can comprise a hydrophobising agent, which covers at least partially the surface of the precipitated calcium carbonate.

The PCC suspension and/or PCC obtained by the process of the present invention may be used in various materials. According to one embodiment of the present invention, the aqueous suspension of precipitated calcium carbonate according to the present invention is used in aqueous formulations, e.g. in a coating formulation, a paper coating formulation, a pharmaceutical formulation, an agricultural formulation, a cosmetic formulation or an oral care formulation. According to another embodiment of the present invention, the precipitated calcium carbonate according to the present invention is used in paper, a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product or a pharmaceutical product.

According to one aspect of the present invention, a product comprising the aqueous suspension of precipitated calcium carbonate according to the present invention is thus provided. According to a preferred embodiment, the product is an aqueous formulation, and more preferably is a coating formulation, a paper coating formulation, a pharmaceutical formulation, an agricultural formulation, a cosmetic formulation or an oral care formulation.

According to another aspect of the present invention, a product comprising the precipitated calcium carbonate according to the present invention is provided. According to a preferred embodiment, the product is a paper, a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product or a pharmaceutical product.

According to still a further aspect of the present invention, a dried precipitated calcium carbonate is provided, which is obtainable by a process comprising the steps of:

-   -   i) providing a calcium oxide containing material,     -   ii) providing at least one copolymer of the following formula         (I)

-   -   wherein x, y and z are present in blocks, alternating or         randomly; x is >0 and at least one of y or z is >0 and the sum         of x+y+z is ≤150; R₁ represents hydrogen or a sulphonic         functional group; R₂ represents a heteroatom, optionally         substituted with an alkyl group, an alkenyl group, an heteroaryl         group and/or a polyalkoxylated group; R₃ and R₄ are         independently from each other a hydroxyl group, (O⁻M⁺) with M⁺         being a monovalent, divalent or trivalent cation, an O-alkyl         group comprising from 1 to 20 carbon atoms, an N-alkyl group         comprising from 1 to 20 carbon atoms and/or a polyalkoxylated         group,     -   iii) preparing a milk of lime by mixing water, the calcium oxide         containing material of step i), and the at least one copolymer         of step ii) to obtain a milk of lime, wherein the calcium oxide         containing material and the water are mixed in a weight ratio         from 1:1 to 1:12,     -   iv) carbonating the milk of lime obtained from step iii) to form         an aqueous suspension of precipitated calcium carbonate,     -   v) optionally, adding at least one slaking additive to process         step iii),     -   vi) separating the precipitated calcium carbonate from the         aqueous suspension obtained from step iv), and     -   vii) drying the separated precipitated calcium carbonate         obtained from step vi).

Optionally, the obtained dried precipitated calcium carbonate can comprise a hydrophobising agent, which covers at least partially the surface of the precipitated calcium carbonate.

According to a preferred embodiment, the dried precipitated calcium carbonate obtainable from process steps i) to vii) is a dried powder of precipitated calcium carbonate.

The dried PCC obtainable from process steps i) to vii) may be used in paper, plastics, polymer compositions, paint, coatings, concrete, cosmetics, pharmaceutics and/or agriculture applications. According to a preferred embodiment, the dried precipitated calcium carbonate is used in plastics and/or polymer compositions. For example, said PCC may be used in thermoplastic polymers, such as polyvinyl chloride, polyolefins, and polystyrene. Moreover, the dried PCC may also be used in polymer coatings which may be applied on the surface of polymer articles, such as foils, in order to increase the hydrophobicity (e.g., reflected by an increased contact angle measured against water) of said surface.

According to one aspect of the present invention, a product comprising dried precipitated calcium carbonate according to the present invention, preferably a dried powder of said precipitated calcium carbonate, is provided. According to one embodiment, the product is a paper, a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product or a pharmaceutical product. According to a preferred embodiment, a product comprising a dried precipitated calcium carbonate is provided, wherein the product is a plastic or a polymer composition.

The scope and interest of the present invention will be better understood based on the following figures and examples which are intended to illustrate certain embodiments of the present invention and are non-limitative.

DESCRIPTION OF THE FIGURE

FIG. 1 is a sketch of a continuous slaking process.

EXAMPLES 1. Measurement Methods

In the following, measurement methods implemented in the examples are described.

Brookfield Viscosity

The Brookfield viscosity of the aqueous suspensions was measured one hour after the production and after one minute of stirring at 25° C.±1° C. at 100 rpm by the use of a Brookfield viscometer type RVT equipped with an appropriate disc spindle, for example spindle 2 to 5.

pH Value

The pH of a suspension or solution was measured at 25° C. using a Mettler Toledo Seven Easy pH meter and a Mettler Toledo InLab® Expert Pro pH electrode. A three point calibration (according to the segment method) of the instrument was first made using commercially available buffer solutions having pH values of 4, 7 and 10 at 20° C. (from Sigma-Aldrich Corp., USA). The reported pH values are the endpoint values detected by the instrument (the endpoint was when the measured signal differed by less than 0.1 mV from the average over the last 6 seconds).

Particle Size Distribution

The particle size distribution of the prepared PCC particles was measured using a Sedigraph 5120 or 5100 from the company Micromeritics, USA. The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement was carried out in an aqueous solution comprising 0.1 wt.-% Na₄P₂O₇. The samples were dispersed using a high speed stirrer and supersonics. For the measurement of dispersed samples, no further dispersing agents were added.

Solids Content of an Aqueous Suspension

The suspension solids content (also known as “dry weight”) was determined using a Moisture Analyser MJ33 from the company Mettler-Toledo, Switzerland, with the following settings: drying temperature of 160° C., automatic switch off if the mass does not change more than 1 mg over a period of 30 sec, standard drying of 5 to 20 g of suspension.

Specific Surface Area (SSA)

The specific surface area was measured via the BET method according to ISO 9277 using nitrogen, following conditioning of the sample by heating at 250° C. for a period of 30 minutes. Prior to such measurements, the sample is filtered within a Büchner funnel, rinsed with deionised water and dried overnight at 90 to 100° C. in an oven. Subsequently the dry cake is ground thoroughly in a mortar and the resulting powder placed in a moisture balance at 130° C. until a constant weight is reached.

Specific Carbonation Time

The monitoring of the conductivity, which slowly decreases during the carbonation reaction and rapidly decreases to a minimal level, thereby indicating the end of the reaction, was used to assess the time needed to perform the complete precipitation. The specific carbonation time (min/kg Ca(OH)₂) was determined by the following formula:

${{Specific}\mspace{14mu}{carbonation}\mspace{14mu}{time}} = \frac{10^{5} \cdot T_{f}}{M \cdot {SC}_{MoL}}$ wherein:

-   -   T_(f)(min) is the time needed to complete the carbonation of the         milk of lime, as determined by monitoring the conductivity,     -   M (g) is the weight of the milk of lime introduced into the         carbonation reactor, and     -   SC_(MoL) (%) is the weight solids content of the milk of lime.         Charge Measurement—Mütek

The charge measurement was carried out using a Mütek PCD 03 device equipped with a Mütek PCD titrator.

About 1 g of the PCC suspension is weighed in the plastic measuring cell and is diluted with 20 mL of deionised water. Put the displacement piston on. While the piston oscillates in the cell, wait until the streaming current between the two electrodes stabilizes.

The sign of the measured value shown on the display indicates whether the charge of the sample is positive (cationic) or negative (anionic). An oppositely charged polyelectrolyte of known charge density is added to the sample as a titrant (either sodium polyoxyethylene sulphate 0.001 N or pDADMAC 0.001 N). The titrant charges neutralize existing charges of the sample. Titration is discontinued as soon as the point of zero charge (0 mV) is reached.

Titrant consumption in mL forms the basis for further calculations. The specific charge quantity q [μVal/g of slurry] is calculated according to the following formula: q=(V×c)/m V: consumed titrant volume [l] c: titrant charge concentration [μVal/l] m: mass of the weighed slurry [g] q: specific charge quantity [μVal/g of slurry] Zeta Potential

For measuring the Zeta potential, a few drops of the PCC suspensions are dispersed in a sufficient quantity of serum obtained by mechanical filtration of the said suspension in order to obtain a colloidal suspension which is slightly cloudy.

This suspension is introduced into the measuring cell of the Zetasizer Nano-ZS from Malvern, which directly displays the value of the Zeta potential of the PCC suspension in mV.

2. Polymers and Slaking Additives

NaCi: Sodium citrate, commercially available from Sigma-Aldrich, Switzerland.

P1: Copolymer of the following formula (III)

-   -   wherein x and z are present alternating; x and z are         independently from each other each an integer in the range from         1 to 149 and the sum of x+z is an integer in the range from 2 to         150; R₁ represents hydrogen; R₃ is (O⁻M⁺) with M⁺ being sodium;         and M⁺ being sodium, and the molar ratio of x to z is about 1:1         (M_(w)=5 000 g/mol; solids content of 30 wt.-%)

P2: polyacrylic acid with the following formula,

-   -   wherein R₁ is H, X is Na, and m=45; the M_(w) being 4 270 g/mol,         and the polydispersity index being 2.3. The molecular weight         M_(w) and the polydispersity index are determined according to         the corresponding method described in EP 14 166 751.9.

3. Examples Example 1

A milk of lime was prepared by mixing under mechanical stirring water with dry sodium citrate (NaCi) as slaking additive (if present) and copolymer P1 (if present) (according to the invention) or polymer P2 (comparison) at an initial temperature between 40 and 41° C. (the amounts of slaking additives and copolymer or polymer are indicated in Table 1 below). Subsequently, calcium oxide (quicklime raw material from Golling, Austria) was added. The obtained mixture was stirred for 25 min and then sieved through a 200 μm screen.

The obtained milk of lime was transferred into a stainless steel reactor, wherein the milk of lime was cooled down to 50° C. Then the milk of lime was carbonated by introducing an air/CO₂ mixture (26 vol-% CO₂ at a rate of 23 L/min). During the carbonation step, the reaction mixture was stirred with a speed of 1 400 rpm. The kinetic of the reaction was monitored by online pH and conductivity measurements.

TABLE 1 Characteristics of produced milks of lime of Example 1 (comp: comparative example; IN: inventive example). Slaking Solids Polymer additive content amount amount of milk Polymer [wt.-%/wt. Slaking [wt.-%/wt. of lime Sample additive CaO] additive CaO] [wt.-%] 1 (comp) No — NaCi 0.1 25.2 2 (comp) No — NaCi 0.1 16.2 3 (IN) P1 0.15 NaCi 0.1 26.9 4 (IN) P1 0.15 — — 27.1 5 (comp) P2 0.15 NaCi 0.1 29.5

The characteristics of the prepared milks of lime and aqueous PCC suspensions are described in Table 2 below.

TABLE 2 Characteristics of the obtained aqueous PCC suspensions of Example 1 Viscosity Solids of the content Viscosity Mutek milk of Carbonation of the of the charge lime time PCC PCC Zeta (μVal/g (mPa · s) (min/kg (wt.- (mPa · s) SSA d₅₀ potential of Sample 100 rpm Ca(OH)₂) %) 100 rpm [m²/g] [μm] (mV) slurry) 1 Viscosity of milk of lime is too high Not measured (comp) 2 23 44 20.5 20 4.7 1.6 +5.5 +0.1 (comp) 3 (IN) 150 46 35.0 199 4.2 1.5 −18.1 −0.5 4 (IN) 101 51 34.2 1730 4.5 1.7 +3.1 +3.4 5 329 47 37.6 940 5.0 1.3 −35.2 −0.9 (comp) (comp: comparative example).

The results compiled in Table 2 show that the use of a slaking additive alone leads to a milk of lime (calcium hydroxide suspension) having a high Brookfield viscosity (sample 1), and it is not possible to increase the solids content of the milk of lime without an increase in the viscosity of the suspension (comparison of sample 1 and sample 2).

By contrast, inventive sample 3 confirms that the viscosity of the obtained milk of lime and PCC suspension is totally compatible with the intended use of the PCC so obtained that is to say suspensions of PCC having a Brookfield viscosity of less than or equal to 1 500 mPa·s at 25° C. Additionally, the kinetic of carbonation and the crystallographic structure of the prepared PCC (results not shown) is similar to the one obtained with a process involving the use of an anionic polymer (P2: polyacrylic acid where 100 mole-% of the carboxylic groups have been neutralized by sodium ions, the M_(w) being 4270 g/mol, and the polydispersity index being 2.3; sample being outside of the invention). 

The invention claimed is:
 1. A process for producing an aqueous suspension of precipitated calcium carbonate comprising the steps of: i) providing a calcium oxide containing material; ii) providing at least one copolymer of the following formula (I):

wherein x, y and z are present in blocks, alternating or randomly; x is >0 and at least one of y or z is >0 and the sum of x+y+z is <150 and wherein a molar ratio of x to y and/or z in the at least one copolymer of step ii) [x:y and/or z] is from 10:1 to 1:2; R₁ represents hydrogen or a sulfonic functional group; R₂ represents a heteroatom, optionally substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group; R₃ and R₄ are, independently from each other, a hydroxyl group, (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation, an O-alkyl group comprising from 1 to 20 carbon atoms, an N-alkyl group comprising from 1 to 20 carbon atoms and/or a polyalkoxylated group; iii) preparing a milk of lime by mixing water, the calcium oxide containing material of step i), and the at least one copolymer of step ii) to obtain a milk of lime, wherein the calcium oxide containing material and the water are mixed in a weight ratio from 1:1 to 1:12; and iv) carbonating the milk of lime obtained in step iii) to form an aqueous suspension of precipitated calcium carbonate with a clustered scalenohedral crystal structure.
 2. The process of claim 1, wherein y>0 and z>0.
 3. The process of claim 1, wherein the calcium oxide containing material and the water are mixed in a weight ratio of from 1:2.5 to 1:5.
 4. The process of claim 1, wherein step iii) comprises the steps of: a1) mixing the at least one copolymer of step ii) with water, and a2) adding the calcium oxide containing material of step i) to the mixture of step a1); or b1) mixing the calcium oxide containing material of step i), and the at least one copolymer of step ii), and b2) adding water to the mixture of step b1); or c) mixing the calcium oxide containing material of step i), the at least one copolymer of step ii) and water simultaneously.
 5. The process of claim 1, wherein the process further comprises step v) of adding at least one slaking additive to process step iii).
 6. The process of claim 1, wherein the milk of lime obtained in step iii) has a Brookfield viscosity from 1 to 1 000 mPa·s at 25° C.; and/or the suspension of precipitated calcium carbonate obtained in step iv) has a Brookfield viscosity of less than or equal to 3 000 mPa·s at 25° C.
 7. The process of claim 6, wherein at least one of: the milk of lime obtained in step iii) has a Brookfield viscosity from 5 to 800 mPa·s at 25° C.; and the suspension of precipitated calcium carbonate obtained in step iv) has a Brookfield viscosity of less than or equal to 2 500 mPa·s at 25° C.
 8. The process of claim 1, wherein the suspension of precipitated calcium carbonate obtained in step iv) has solids content of at least 15 wt.-%, based on the total weight of the suspension.
 9. The process of claim 8, wherein the suspension of precipitated calcium carbonate obtained in step iv) has a solids content of no more than 70 wt.-%, based on the total weight of the suspension.
 10. The process of claim 1, wherein the temperature of the water, which is used in mixing step iii), is adjusted to be in the range of from more than 0° C. to less than 100° C.; and/or the temperature of the milk of lime obtained in step iii), which is employed in step iv), is adjusted to be in the range from 20° C. to 65° C.
 11. The process of claim 1, wherein the at least one copolymer of step ii) is a compound of the following formula (II):

wherein x and y are present in blocks, alternating or randomly; x and y are >0 and the sum of x+y is <150; R₁ represents hydrogen or a sulfonic functional group; and R₂ represents a heteroatom, optionally substituted with an alkyl group, an alkenyl group, an heteroaryl group and/or a polyalkoxylated group.
 12. The process of claim 1, wherein the at least one copolymer of step ii) is a compound of the following formula (III):

wherein x and z are present in blocks, alternating or randomly; x and z are >0 and the sum of x+z is <150; R₁ represents hydrogen or a sulfonic functional group; R₃ is a hydroxyl group, (O⁻M⁺) with M⁺ being a monovalent, divalent or trivalent cation, an O-alkyl group comprising from 1 to 20 carbon atoms, an N-alkyl group comprising from 1 to 20 carbon atoms and/or a polyalkoxylated group.
 13. The process of claim 1, wherein the molar ratio of x to y and/or z in the at least one copolymer of step ii) [x:y and/or z] is from 5:1 to 1:2.
 14. The process of claim 1, wherein the at least one copolymer of step ii) has a molecular weight M_(w), in the range from 500 to 100,000 g/mol; and/or, provided in a water solution, has a Brookfield viscosity of less than or equal to 5000 mPa·s at 25° C.
 15. The process of claim 14, wherein the at least one copolymer of step ii) has a molecular weight M_(w), in the range from 1000 to 50,000 g/mol; and/or, provided in a water solution, has a Brookfield viscosity of less than or equal to 2000 mPa·s at 25° C.
 16. The process of claim 1, wherein the process further comprises, after step iv), a step viii) of contacting at least a part of the surface of the obtained precipitated calcium carbonate with at least one hydrophobising agent wherein the at least one hydrophobising agent is selected from the group consisting of an aliphatic carboxylic acid having a total amount of carbon atoms from C₄ to C₂₄ and/or reaction products thereof, a mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from at least C₂ to C₃₀ in the substituent and/or reaction products thereof, a phosphoric acid ester blend of one or more phosphoric acid mono-ester and/or reaction products thereof and one or more phosphoric acid di-ester and/or reaction products thereof, polyhydrogensiloxane and reaction products thereof, an inert silicone oil, an aliphatic aldehyde having from 6 to 14 carbon atoms and/or reaction products thereof, and mixtures thereof.
 17. The process of claim 1, wherein the process further comprises step vi) of separating the precipitated calcium carbonate from the aqueous suspension obtained in step iv), and optionally step vii) of drying the separated precipitated calcium carbonate obtained in step vi).
 18. Precipitated calcium carbonate with a clustered scalenohedral crystal structure obtained by a process according to claim
 17. 19. A product comprising precipitated calcium carbonate with a clustered scalenohedral crystal structure according to claim 18, wherein the product is selected from the group consisting of: a paper product, an ink, a paint, a coating, a plastic, a polymer composition, an adhesive, a building product, a foodstuff, an agricultural product, a cosmetic product, and a pharmaceutical product. 